1. Field of the Invention
The present invention relates to a sheet light emitting apparatus configured to introduce light from a light source into a light guiding plate, emit the light through the light guiding plate and to illuminate all of a surface of an object to be illuminated such as a liquid crystal cell in a liquid crystal display, by the exit light from the light guiding plate.
2. Description of Related Art
A thinned liquid crystal display is used recently, as a display of a book type word processor, a computer, a mobile phone or other mobile terminal devices. A sheet light emitting apparatus having a light guiding plate is used in order to illuminate such a thinned liquid crystal display. A line shaped light source such as a fluorescent tube and a point-shaped light source comprising a plurality of light emitting diodes (hereinafter, referred to as LEDs) or the like are used, as a light source of the sheet light emitting apparatus. The LEDs have a low applied voltage and lower power consumption and therefore are used as a light source in a small liquid crystal display or the like.
Recently, more bright illumination has been requested to the light source, because of evolution of colorization in the small liquid crystal display, and multi brightness of a conventional liquid crystal display in black and white has been desired. Therefore, light having a high directivity, which is emitted from the LEDs as the light source is used and it has been required to take the light into a light guiding plate having a more high density. Accordingly, in the sheet light emitting apparatus using the plurality of LEDs as the light source, LEDs having a high directivity of emitted light are used. However, when the LEDs having the high directivity of the emitted light are used, there is a defect that a valley of light for propagating within a light guiding plate 31 is generated in a side (side of an end surface 31a) of the light guiding plate 31 close to the LEDs 32, for example, as sown in Japanese Patent Laid-Open No. 2002-082625 (FIG. 4) and therefore there is generated a triangle A to which light does not arrive between the LEDs and both ends of the LEDs.
FIGS. 13A to 13C illustrate one example of a conventional sheet light emitting apparatus. In FIGS. 13A to 13C, reference numeral 102 denotes LEDs constituting a light source for emitting light, 101 a light guiding plate, 103 a prism sheet disposed to face an upper surface 101a of the light guiding plate 101, 104 a reflective sheet disposed adjacently to face a lower surface 101b of the light guiding plate 101.
As shown in FIG. 13A, the light guiding plate 101 is a rectangular in a planar shape and is made of a translucent material of a plastic material or the like. The lower surface 101b of the light guiding plate is formed into a scattering surface of light of uneven concave and convex surfaces by crimps, dots, prisms or printing.
A plurality of LEDs 102 are disposed to face a light receiving surface 101c which is one side surface of the light guiding plate 101. Four surfaces of the light guiding plate 101, including the upper and light receiving 101a and 101c are formed into smooth surfaces of mirror surfaces or the like.
When applying a predetermined current to the LEDs 102 to light them, light emitted from the LEDs 102 enters passing through the light receiving surface 102c by refracting into the light guiding plate 101 and becomes inner light which widens within the light guiding plate. The inner light, while repeating diffused reflection on the lower surface 101b of the light guiding plate 101 and total reflection on the upper surface 101a thereof, propagates within the light guiding plate 101, and transmits the upper Surface 101a by refracting during the propagation, and exits toward above. The exited light enters into the prism sheet 103, a direction is arranged substantially perpendicularly on the drawing by refracting operation of the prism sheet to become planer illumination light, and the planar illumination light is emitted from the prism sheet toward an illuminated object (not shown) such as a liquid crystal panel.
In addition, the reflective sheet 104 operates to reflect light emitted outwardly by scattering of light on the lower surface 101b of the light guiding plate 101, to return in the light guiding plate 1 again, and to improve the usability of light.
Here, an area S shown at diagonal lines in FIG. 13A is a light present area in which the inner light entering from the LEDs 102 passing through the light receiving surface 101c with refraction into the light guiding plate 101 is existing and distributed within the light guiding plate 101. An area S1, which has no diagonal lines is a light absent area in which the inner light is not existing. The light absent area S1 that the inner light is not existing forms a triangle having a hypotenuse which corresponds to refracting light of the maximum refracting angle. As shown in FIG. 18C, for example, when the maximum exit angle of light s emitted from the LEDs 102 is 55°, the maximum refracting angle or the maximum exit angle of the refraction in the light receiving surface 101c in which the light s enters becomes about 30° and an angle formed between the hypotenuse and a bottom of the triangle of the light absent area S1 is about 60°.
In this way, the light absent area S1 that the inner light does not exist in the light guiding plate 101 exists in a state entering considerably from the light receiving source 101c. Accordingly, in the sheet light emitting apparatus using the light guiding plate 101, there are a bright part and a dark part alternately in the portion near the LEDs 102 or portion near the light receiving surface 101c, when illuminating the illuminated object such as a liquid crystal cell, the brightness is uneven and a good illumination is not required. The unevenness of brightness has a problem especially because the distribution of brightness in the light guiding plate reflects almost direct to the brightness of the illumination light in a structure in which the prism sheet 103 is disposed to face directly the light guiding plate without providing a diffusion plate between the light guiding plate and the prism sheet, because of a thinned type and a simplification for the sheet light emitting apparatus, as shown in FIGS. 13A to 13C.
To improve the problem, for example, as shown in Japanese Patent Laid-Open 2002-196151 (FIGS. 1 and 2), there is known a sheet light emitting apparatus in which a plurality of prisms are formed on a light receiving surface 1c of a light guiding plate 1 facing LEDs 2. According to this, because an incident angle of light from the LEDs into the prisms can be small, a less turning of light from the LEDs due to refraction when the light enters into the light guiding plate can be accomplished, an area of a dark part of illumination can be reduced and uneven brightness of illumination can be improved.
FIGS. 14A to 14C illustrate a conventional sheet light emitting apparatus having a light guiding plate provided with the prisms as described in FIGS. 14A to 14C, reference numeral 101p denotes a plurality of prisms provided on a light receiving surface 101c of the light guiding plate 101. The other structure is the same as that of the sheet light emitting apparatus as shown in FIG. 13A to C.
An apex angle α of each of the prisms 101p as shown in FIG. 14C is less than 60° in one example. At this time, the light s emitted at the maximum exit angle 55° from the LEDs 101 is entered at a side upper than a normal line to an oblique surface of each of the prisms 101p and turns outwardly by the refraction and as a result, the exit angle of transmitted light sp to the light receiving surface 101c is more than 55°. The exit angle is about 60° in the example as shown in the drawings.
If the apex angle of the each prism 101p is 70°, the incident angle of the light s to the prism becomes 0°, and the light goes directly without turning and the exit angle of the transmitted light sp is 55° while the refracted light in which the apex angle α is less than 70°, turns outwardly, and the exit angle of the transmitted light sp is more than 55°, if the apex angle α is less than 60°, the exit angle approaches 60°. In other words, the maximum exit angle standardizing the light receiving surface 102c of the transmitted light sp is about 60°, which is larger than the maximum exit angle, about 30°, in the absence of the prisms, as shown in FIG. 13C.
When the maximum exit angle is small, as shown in FIG. 13A, a gradient of an oblique plane of a triangle of a light absent area S1 is sharp and the entrance of light into the light guiding plate 102 at the light absent area S1 is deep.
On the contrary, when the maximum exit angle is large, as shown in FIG. 14A, the gradient of the oblique plane of the triangle of the light absent area S1 is gentle and the entrance of light into the light guiding plate 102 at the light absent area S1 is shallow. The entrance of light into the light absent area S1 becomes more shallow, the uniformity of brightness of light in the light guiding plate is increased.
Here, the maximum exit angle within the light guiding plate 101 is further large and the entrance of light into the light absent area S1 can be shallowed, as the apex angle of each prism becomes small and the inclined angle of the oblique plane of each prism becomes large. Consequently, it is possible to shallow the entrance of light into the light guiding plate 101 at the light absent area S1, to enhance the uniformity of brightness of inner light in the light guiding plate and to thus enhance the uniformity of brightness of illumination light, by selecting suitably the apex angles of the prisms as described above.
In addition, FIG. 15 illustrates collecting light fluxes in which the transmitted lights of the prisms are collected. In FIG. 15, reference sign SK denotes the collecting light fluxes in which a plurality of transmitted lights sp of the prisms 101p sown in FIG. 14C are collected every the LEDs 102. FIG. 16A illustrates a distribution of the collecting fluxes SK in the light guiding plate 101.
There is a problem in the sheet light emitting apparatus using the light guiding plate having the plurality of prisms as follows.
That is to say, as described above, lights emitted from the LEDs transmit the plurality of prisms 101p to form a plurality of transmitted lights sp, the transmitted lights sp are collected to form the collecting light fluxes SK and then the collecting light fluxes are emitted from each of the LEDs 102 as shown in FIG. 16A, separate rightward and leftward (upward and downward on the drawings into approximately mountain shapes and go and distribute in the light guiding plate 101. In the collecting light fluxes SK, when spaces (w in FIG. 15) between the transmitted lights of the prisms are narrow, the collecting light fluxes SK are recognized as one combined light to distinguish as a bright line.
Meanwhile, dark parts are easy to occur in periphery (gaps between the collecting light fluxes) of the collecting light fluxes SK.
That is to say, as shown in FIG. 15, if the maximum exit angles of the transmitted lights sp are large, and the minimum oblique angles of the prisms to the light receiving surface 101c are small, the space W between the adjacent transmitted lights with respect to the transmitted lights sp refracted by the plurality of prisms 101p is small, there are cases that the space can be discriminated by the naked eye and the plurality of light fluxes are seen together.
In such a case, as shown in FIG. 16A, each of the transmitted lights sp (see FIG. 15) of the prisms in the collecting light fluxes SK of the mountain shapes widening rightward and leftward (upward and downward on the drawing) starting from each of the LEDs 102, as a whole of the light guiding plate 101 cannot be discriminated, and therefore the collecting light fluxes SK are recognized as one light, whereby distinguishing as the bright line.
Next, despite whether the collecting light fluxes SK are recognized as one body, there are areas R of gaps between the collecting light fluxes SK as shown in FIG. 16A, the brightness of light tends to drop in each of the areas. According to this, the drop of brightness in each of the areas R of gaps is seen, while, in particular, a significant drop of the brightness in a B—B section is seen (see FIG. 16B).
In this way, in the conventional sheet light emitting apparatus having the light guiding plate provided with the prisms, the entrance of light into the light guiding plate 101 at the light absent area S1 is shallow by means of each the prisms 101p, there is an advantageous effect in this point but there is a case that the bright line of the collecting light fluxes as described above is distinguishing in the light guiding plate 101 and there is a problem that the formality of brightness is reduced within the light guiding plate.
Meanwhile, in the sheet light emitting apparatus having no diffusion plate as shown in FIGS. 14A to 14C, because the distribution of brightness of the inner light in the light guiding plate and the generation of the bright line are reflected directly on the quality of illumination light, there is generated a large problem in particular. In other words, in the sheet light emitting apparatus as shown in FIG. 17, which is a reference drawing, a diffusion sheet 105 is disposed to face a light guiding plate 101 and prism sheets 103 are disposed above the diffusion sheet 105. In this case, a bright line of inner light and a distribution of brightness in the light guiding plate 101 are smoothed to a degree and reflected on illumination light.
However, in the sheet light emitting apparatus as shown in FIG. 14B, and explained already, in which the prism sheet 103 is disposed to face above the light guiding plate 102 without providing the diffusion sheet above the light guiding plate for the purpose of the simplification and thinned type, visibility of the bright line and the drop portion of the brightness (R in FIG. A) are easy directly to reflect on the illumination light, and therefore a state of illumination is aggravated by the visibility of bright line and the drop portion of brightness.